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The present invention relates to a method for producing a liquid film using a release agent and a release liquid which are used for the manufacture of adhesive labels, peel-off label stickers, double-sided tapes, etc. In particular, the present invention relates to a method for producing a liquid film comprising the steps of obtaining a release liquid by dispersing a mixture of an aqueous emulsion silicone release agent and a modified colloidal silica in water and using the release liquid as a release layer.
In general, as shown in FIG. 1, a release film is produced by applying a release liquid as a release layer 2 on a substrate 1 such as a plastic film, a sheet, a paper or a non-woven fabric at a predetermined thickness. For example, U.S. Pat. Nos. 5,672,428 and 5,728,339 disclose a release layer obtained by forming a plastic film or sheet as a substrate, followed by applying a release agent thereon.
However, conventional release films using the release layer have disadvantages of inadequate peel strength and adhesive strength. That is, a peel strength sufficient to release a release film from an adherent after the release film is contacted with the adherent results in weak adhesive strength between the substrate and the release agent. In addition, the release film causes problems such as peeling off of the release agent from the substrate and poor solvent resistance.
There have been known a number of methods for improving the adhesion between a substrate and a release agent. For example, when a plastic film is used as the substrate, the surface of the plastic film is chemically treated with chromic acid, ozone, corona, flame, radioactive ion beam, etc. In particular, when the plastic film is a polyester film, the surface of the plastic film is treated with a highly polar polymer such as modified polyester having a high affinity, a partially hydrolyzed ethylene-vinyl acetate copolymer, polyvinylbutyral, etc.
In addition, methods for improving the peel strength between a release layer and an adherent, and the adhesion between a substrate and the release layer have been known. For example, U.S. Pat. No. 5,298,325 suggests a method for forming a release layer by forming a primer layer on a substrate, followed by off-line coating on the primer layer. However, the patent has problems in terms of complex processes including the off-line coating, and increased manufacturing cost.
In common release films, increasing of the adhesive strength between a release layer and a substrate results in low peel strength, and increasing of peel strength to improve the releasability results in low adhesive strength between the release layer and the substrate.
According to U.S. Pat. Nos. 5,672,428 and 5,728,339, a release film is produced by in-line coating a plastic substrate once with a silicone aqueous emulsion release agent. At this time, an adhesion promoter is added to the release agent to improve adhesion between the film and the release agent. Examples of the adhesion promoter used herein include water-dispersible polyester, water-dispersible acrylic resin, glycidoxyalkoxysilane, etc.
A stretching operation must be performed during the in-line coating of the plastic film with the release agent. At this time, the operation requires considerable care so as not to cause poor adhesion between the release agent and the substrate, and smoothness of the release layer. When in-line coating the plastic substrate with the release agent, poor smoothness of the release layer results from irregular contraction and expansion of the release layer during stretching or annealing operation following coating the release agent, or from partial crosslinking caused by different cross linking rates due to non-uniform heat transfer between the release layer and the substrate.
On the other hand, most conventional in-line coating processes are done at a certain step during the manufacture of the plastic film as a substrate. Accordingly, the release properties of a silicone release film largely depend on the manufacturing stage and properties of the film itself. As a result, when peeling a laminate of the release film and an adherent, the release properties do not always show isotropy according to a physical force to peel the release film and touch by a person. Since most biaxially stretched films are manufactured by a sequential stretching process, minimization of differences between physical properties in both machinery and transverse directions is difficult to accomplish. In particular, when the differences between the physical properties in both the directions of the film exceed a certain range, they affect release properties of the release layer applied on the film.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a release film having an excellent adhesive strength between a substrate and a release agent, and having excellent peel strength between the release film and an adherent.
It is another object of the present invention to provide a simplified manufacturing process of the release film by using a silicone aqueous emulsion release agent containing a vinyl-modified colloidal silica, instead of a release agent containing an organic solvent, thereby causing no volatilization of organic solvent during producing the release film, and forming the release layer by in-line coating.
It is yet another object of the present invention to provide a release film having excellent isotropic release properties by improving the uniformity of the substrate (e.g., polyester film) so as to have uniform release properties regardless of peeling directions (see, FIG. 3), when the release film is released from the adherent (see, FIG. 2).
In accordance with the present invention, there is provided a method for producing a release film comprising the steps of : obtaining a release liquid by dispersing a silicone aqueous emulsion release agent including 1xcx9c10% by weight of a dispersion of a vinyl-modified colloidal silica having a solid content of 10xcx9c40% by weight, 0.1xcx9c5% by weight of a dispersion of a platinum chelate catalyst having a solid content of 12xcx9c25% by weight and the remaining amount of a polysiloxane water-dispersible silicone emulsion having a solid content of 30xcx9c60% by weight, in water, so as to achieve a solid content of the release liquid of 2.5xcx9c30% by weight; and applying and drying the obtained release liquid on a substrate to form a release layer.